1. Field of the Invention
This invention relates generally to coding techniques utilized in connection with spread spectrum radar systems and more particularly to a secure spread spectrum (SS) encoding technique which merges communication and radar technologies, whereby an airborne radar interrogates and receives pseudo noise coded messages from one or more ground based digital RF tags.
2. Description of Related Art
Spread spectrum is a well known modulation technique wherein a transmitted RF signal is spread over a wide frequency band and has particular applicability not only in communication systems, but also in the field of radar where there is a need for avoiding detection by countermeasure systems employed either on the ground or by other aircraft. This technique is typically utilized in connection with synthetic aperture radar (SAR) and ground mapping target indicator (GMTI).
Several general types of spread spectrum techniques are known. One type is known as direct sequence modulation and involves modulation of a carrier by a digital code sequence whose bit rate is much higher than the information signal bandwidth. The second type employs FM modulation called “chirp” wherein a carrier is swept over a wide band during a given pulse interval. The third type involves carrier shifting or hopping in discrete increments in accordance with a predetermined code sequence.
Thus in all cases, spread spectrum transmission thus involves expanding the bandwidth of an information signal, transmitting the expanded signal, and then recovering the desired information signal by remapping the received spread spectrum original information signal's bandwidth.